1. Field of the Invention
The present invention relates to an apparatus and method for printing, and more particularly, to an apparatus and method for replacing printing plates on a plate cylinder contained within a printing unit, while the printing unit is continuously printing on a web of material.
2. State of the Art
In commercially available web fed rotary printing presses, a plurality of printing units can be arranged one above the other to print on one or more paper webs which pass through the printing units in a substantially vertical direction. Printing units typically include multiple rolls that apply ink to one or both sides of the paper webs. An ink train includes multiple rolls in contiguous contact, to transport ink to printing plates of each printing unit. The ink train meters ink supplied to the printing plates, so that the ink is provided as a uniform film of a desired thickness.
A substantial amount of time is required to exchange printing plates. The printing plates are replaced if, for example, a new paper is being printed with a different advertisement in each of different communities. Due to the relatively fixed arrangement of the printing units and a limited amount of space available in a press room, the printing units are not easily accessible by the press operators, such that printing plates can not be easily replaced.
Moreover, even when access to the printing plates is achieved, replacement can be complex. When a printing plate is changed within a printing unit of a printing press, the printing process is stopped within that unit by removing any contact of blanket cylinders with the moving paper web. For example, German Patent DE 195 48 819 describes a stacked printing press arrangement in which a blanket cylinder can be separated from the running web to change a printing plate on an associated plate cylinder. A pivotable guard is provided for protecting the press operator from the rotating cylinders and rollers of another printing unit currently in operation. However, one drawback of this type of printing plate change is that an entire printing unit must be taken out of operation during the printing plate change. Because the entire printing unit must be shut down, an auxiliary printing unit having redundant printing plates is required.
It is known to change printing plates of web fed rotary printing presses "on-the-fly." To perform such a printing plate change, a plate cylinder on one side of the printing press is moved away from the moving web to perform the printing plate change, while the second side of the web is still being printed. For example, German Patent DE 196 03 666 discloses a printing plate change where a plate cylinder is displaced from the blanket cylinder and its rotation is stopped. A guard is inserted into the space created by the displaced plate cylinder to ensure that nothing (e.g. a printing plate or press operator) gets caught in the moving parts of the press. However, the insertion of a safety shield between the cylinders takes up too much space and requires physical movement of the printing carriage (i.e., all cylinders) of the printing unit. Movement of the printing carriage during a printing plate change requires the entire printing unit of a conventional press to be redesigned, and reduces the cost effectiveness of performing a printing plate change on the fly.
British Patent GB 2309668 describes a web offset rotary printing unit in which blanket cylinders remain in contact with the web during a printing plate exchange. A plate cylinder can be moved away from a transfer cylinder to change the printing plates. While the system of GB 2309668 requires only half of a printing unit to be shut down, and requires only half of a redundant printing unit to be used, a large amount of space is allotted between the rotating blanket cylinder and the plate cylinder whose printing plate is being changed. This, in turn, can require movement of the entire ink train and therefore modification of the printing unit. Additionally, there is no mechanism, other than the insertion of a guard, to ensure that a printing plate being removed or placed onto the plate cylinder will not get caught in the moving blanket cylinder.
Accordingly, it would be desirable to provide a method and apparatus for performing a printing plate change on the fly within a conventional printing unit which does not require movement of the entire printing carriage. Additionally, it would be desirable to reduce the space between the blanket cylinder and the plate cylinder whose printing plate is being changed, while continuing to ensure that nothing gets caught in the moving parts of the printing unit.